Self-aligned bipolar transistor structures are known in the prior art. The two basic types of self aligned bipolar transistor structures commonly used in the industry are those with spacers outside the emitter and those with spacers inside the emitter. The structures with spacers outside the emitter generally use one polysilicon layer for the emitter formation and are known as single-poly self-aligned transistors, which resemble self-aligned poly-gate MOS devices. These structures are not very pertinent to the present invention and therefore are not discussed in detail.
On the other hand, the structures with spacers inside the emitter are related to the present invention and will be discussed in more detail. These structures generally use two polysilicon layers and are known as double-poly self-aligned transistors. A general discussion of these transistors is provided by David J. Roulston, BIPOLAR SEMICONDUCTOR DEVICES, Chapter 14, pp. 340-367, published by McGraw-Hill, Inc., (1990).
Generally, double-poly self-aligned transistors use spacers not only to separate the emitter from the base electrode but also to reduce the effective emitter width to a dimension smaller than minimum critical dimensions (CDs) allowed by lithographic constraints.
An example of the prior art structure is shown in FIG. 1. The main feature of this structure is that the base 1 of the transistor is placed in contact with a polysilicon electrode 2 which overlaps the field oxide layer 3. Also, the perimeter of emitter 5 is defined by spacers 4 formed along the inside edges of the polysilicon electrode 2.
The emitter 5 may be placed directly in contact with the metal (resulting in the lowest emitter resistance) or by a second poly layer (not shown). This second poly layer is what gives this structure the name double-poly transistor. Also, the base 1 is in contact with the polysilicon electrode 2 overlying the field oxide layer, which reduces the collector-base capacitance and minimizes the silicon base-collector junction area.
Finally, the use of inside spacers 4 results in a smaller emitter than defined by lithographic pattern size.
One problem with prior art structures, such as the one shown in FIG. 1, is that the standard double-poly transistor has a base resistance component which comes from the base polysilicon electrode. This resistance is generally in the range of 50 to 100 ohms/square. This is due to the resistivity of the polysilicon as determined by mobility of carriers present therein and the amount of doping.